The manufacturing of hydraulic cement dates back to the earliest days of the Roman Empire. Pozzolana, a volcanic ash from one of the world's earliest cement kilns, Mount Vesuvius, was mixed with limestone to form a material capable of hardening under water. During the middle ages this ancient Roman art was lost and it was not until the middle of the eighteenth century that natural hydraulic cements were again made by burning mixtures of clay and limestone at high kiln temperatures to produce a clinker which was mixed with water and allowed to set or cure. However, due to the inherent variability associated with natural clay and limestone the exact composition of these natural cements varied widely and performance was unpredictable.
The art became a science in the early nineteenth century when Joseph Aspdin invented a process of carefully proportioning combinations of calcium, silicon, iron and aluminum found in local clay and lime deposits and burning these materials at high temperatures. This patented process resulted in portland cement with more consistent performance named after the stone quarried on the Isle of Portland off the British coast. Portland type cement is still one of the most commonly used structural materials today. In spite of significant advances in the material sciences, even today the basic process for making cement has remained essentially unchanged. Raw materials including limestone, clay, and bauxite are measured and mixed then fired at temperatures in excess of 1500.degree. C. (2700.degree. F.) until a cement "clinker" is formed. The finished clinker is crushed for use as cement and can be mixed with post production ingredients such as gypsum, soluble CaSO.sub.4 anhydride and additional sources of C.sub.2 S, C.sub.3 S and C.sub.3 A to modify properties. Typically, the latter three come from the addition of conventional portland type cement to the clinker.
For convenience of further description, the following standard cement industry abbreviations will be utilized to describe the composition of fired materials:
H--represents water (H.sub.2 O) PA1 C--represents Calcium Oxide (CaO) PA1 A--Aluminum Oxide (Al.sub.2 O.sub.3) PA1 F--represents Ferric Oxide (Fe.sub.2 O.sub.3) PA1 M--represents Magnesium Oxide (MgO) PA1 S--represents Silicon Oxide (SiO.sub.2) PA1 K--represents Potassium Oxide (K.sub.2 O) PA1 N--represents Sodium Oxide (Na.sub.2 O) PA1 S--represents Sulfur Trioxide (SO.sub.3) PA1 Mn--represent Manganese Oxide (Mn.sub.2 O.sub.5) PA1 P--represent Phosphorous Oxide (P.sub.2 O.sub.5) PA1 f--represent fluorine F PA1 cl--represent Chlorine Cl.
Recent advances in our understanding of cement chemistry, the thermal dynamics of cement kiln operation and control, and pioneering breakthroughs in structural analyses using x-ray diffraction crystallography have allowed material scientists and cement manufactures to overcome and minimize many of the variables and problems inherent in cement manufacturing. However, two particularly vexing problems remain to be fully resolved. First, modern commercial cement compositions rely on a mineral composition known as C.sub.3 S silicate and its hydration (water incorporation) rate for early strength. Yet, these compositions inherently contain high concentrations of non-early strength producing C.sub.2 S in their base clinkers which cannot be converted to the more desirable C.sub.3 S. High early strength and rapid setting times relate to the hydration rate of the C.sub.3 S. General purpose portland type cement (usually designated ASTM I) typically contains approximately 50% C.sub.3 S, 25% C.sub.2 S, 12% C.sub.3 A, 8% C.sub.4 AF, 5% C. Thus the total amount of calcium silicates is approximately 75%, with the predominant silicate being C.sub.3 S. The hydration rate of C.sub.3 S and C.sub.2 S significantly differ with the C.sub.2 S component taking up to one year to fully hydrate. Consequently the C.sub.2 S contributes very little or nothing to the early strength of the cement product. This is even further exacerbated if additional C.sub.2 S is added to the clinkered material by supplementation with hydraulic cement during final product formulation. Consequently, the net silicate hydration rate, and therefore the ultimate rate of strength formation, is limited by the C.sub.2 S hydration rate when the aqueous phase (water) is added.
The second perpetual problem associated with all current cement manufacturing processes is the terrible burden placed on the environment. Cement manufacturing is the single most significant source of atmospheric SO.sub.x (sulfur oxides) contamination. Further, other noxious gaseous emissions are exuded by the ton from the reaction conditions within the cement kiln. What is more, great quantities of fossil fuels are burned to power these huge kilns and plumes of silicon and aluminum particulates are generated by the mixing, packaging and shipping of the raw materials and final cementuous products. Many collateral methods have been developed to reduce these pollutants. However, the clinker formation process is still fraught with potentially disastrous environmental consequences.
There are four primary properties of cement and its products that material scientists continually work to improve: high early strengths, rapid setting time, resistance to degradation, and good expansiveness to offset shrinkage. For example, concrete made from portland cement together with sand, gravel or other mineral aggregate, typically undergoes shrinkage upon drying. This shrinkage is undesirable in that, among other reasons, it gives rise to cracks which ultimately weaken the set concrete.
Cracking results from excessive shrinking and high heats of hydration in thickly poured structures (cement and water react chemically and produce heat unlike plaster and water which merely dries). The shrinkage rate can be controlled through increasing the amount of calcium aluminum sulfate in the clinker which expands upon hydration in the presence of free CaO and CaSO.sub.4. Early attempts at reducing cracking and thereby increasing overall strength and resistance to chemical attack resulted in the so-called "calcium alumino sulfate" cements based upon 3CaO, 3Al.sub.2 O.sub.3, CaSO.sub.4, abbreviated as either C.sub.3 A.sub.3 CS or, preferably C.sub.4 A.sub.3 S. The primary characteristic of C.sub.4 A.sub.3 S cements is their expansiveness. Addition of additives such as C.sub.4 A.sub.3 S counteracts shrinkage and may or may not produce cements having early high strength. Examples of these calcium alumino cements can be found in U.S. Pat. No. 3,155,526 (Klein), U.S. Pat. No. 3,860,433 (Ost) and U.S. Pat. No. 4,798,628 (Mills).
Resistance to chemical degradation, water permeability and chlorine attack are qualities that result from improved resistance to cracking and chemical neutralization of reactive species by ingredients within the cement matrix. Resistance to sulfate attack is provided by limiting the C.sub.3 A content to less than 5%, or using novel means to eliminate C.sub.3 A through reactions with CS.
Excepting the Kunbagri patent discussed below, one consistent element of the prior art has been the use of kiln temperatures in excess of 1500.degree. C. This temperature has been believed necessary by those skilled in the art to encourage the production of the desirable stable calcium silicate C.sub.3 S. However, these elevated kiln temperatures which have dominated the sintering process since Mount Vesuvius first erupted have not been without detriment. The temperatures traditionally used to reach sintering temperatures within the kiln result in a significant source of the primary greenhouse gases released during the calcining of CaCO.sub.3 and through the burning of fossil fuels in the kiln. In addition to CO.sub.2, copious amounts of NO.sub.x, and SO.sub.x also emanate from the kiln as the calcining and sintering processes continue. Furthermore, operating industrial kilns within narrow controlled ranges is extremely difficult due to the lack of precise thermal monitoring equipment that can be used in the high particulate environment of a cement kiln. Consequently, any advance in cement manufacturing material science and chemistry that can improve the final product's desired properties, reduce the number of post production ingredients required, and significantly reduce gaseous emissions would be considered an important advance in cement manufacturing.
Perhaps the most significant advance in portland type cement design and chemistry is disclosed in the present inventor's U.S. Pat. No. 4,957,556 patent (Kunbargi). This patent discloses and claims cement compounds formed from what was then an entirely new class of clinkered materials which for the first time contained high concentrations of C.sub.4 A.sub.3 S. At that time, the present inventor was the first to invent associated methods for enriching clinkers to high concentrations of C.sub.4 A.sub.3 S. Broadly stated, this was achieved by adjusting the ratio of reactants in the raw materials and by using x-ray defraction analysis to carefully control kiln temperature to a narrow and specific range of relatively high temperatures below 1500.degree. C. In addition, cement compounds of the Kunbargi '556 patent exhibited increased resistance to sulfate attack due to the concurrent discovery that soluble CaSO.sub.4 anhydride would react with residual C.sub.3 A in the clinker and exogenous C.sub.3 S sources.
However, though a dramatic improvement over the prior art, this earlier cement formulation and production technology still requires the tedious and expensive addition of controlled amounts of soluble CaSO.sub.4 anhydride and exogenous C.sub.3 S to the finished clinker. The exogenous C.sub.3 S present in this hydraulic cement additive also brings with it the undesirable C.sub.2 S silicate which has a slower hydration rate than would optimally be desired to produce an extremely fast high strength early setting cement. Consequently, although a significant advance over the prior art, the cement compositions of the '556 patent still utilize post manufacturing supplementation with two active ingredients and have early strength qualities which are limited by the slow hydration rates associated with the C.sub.2 S in the hydraulic cement supplement.
In spite of these prior art advances in the production of early setting high strength cement, the development of portland type cements having even greater compressive strengths and higher rates of strength development than those presently available would be of great economic benefit to the cement and the construction industries. For example, in the production of pre-cast, pre-stressed, concrete products, a compressive strength on the order of 4000-5000 psi at three hours is often required. Additionally, in the construction and repair of highways, bridges and freeway over-passes many days and even weeks of curing time are required before these structures set to sufficient compressive strengths to support their anticipated loads so that they may be utilized as designed. The resultant delays cost millions of dollars annually in increased transportation costs and shipping delays while critical transportation corridors are shut down waiting for concrete to harden. Moreover, in the construction of concrete buildings, where the cement matrix is cast into forms, it is necessary to allow days of curing time to allow the cement to develop sufficient strength for removal of the forms. This delay results in lost revenues for property owners and inconvenience and storage costs for industrial tenants. Furthermore, because setting rates of portland type cements can be affected by temperature, an early setting, ultra-high strength cement with a lower heat of hydration that would make the production of large complex superstructures possible in extremely low ambient temperature environments would be an even greater contribution to the construction industry.
However, these and other improvements in cement quality should not be made at the expense of the environment. Cement manufacturing is a notoriously environmentally unfriendly process. In the past, the benefits that society has received from cement, mortar and concrete have considerably outweighed the environmental impact. However, a process for making a superior clinkered material than currently known in the art that would significantly reduce gaseous emissions of SO.sub.x, NO.sub.x and CO.sub.x would represent a tremendous industrial and environmental advance.
Accordingly, it is a particular object of the present invention to provide a rapid hardening high early strength portland-type cement composition with an extremely rapid C.sub.2 S hydration rate. Whereas the best cements known in the art can produce compressive strengths within one hour on the order of 3000 psi and on the order of 6000 psi within one day, the cement compositions of the present invention will produce compressive strengths on the order of 5000-7000 psi within one hour, on the order of 7000-8000 psi within one day. The resulting cement compounds will also possess a sulfate resistance of 0.01% at one year without requiring the addition of soluble CaSO.sub.4 to the finished clinker, a water permeability of less than 1 mm in one year, a drying shrinkage of 0.03% at 28 days, a heat of hydration of 70 cal/g at 28 days.
It is a further additional object of the present invention to provide methods for producing rapid hardening high early strength portland-type cement compositions, and compositions so produced, which are particularly well suited for use in pouring large structures, even in cold temperatures. This advantageous quality is derived from a generally low overall rate of hydration resulting from the present invention, where, unlike the prior art hydration, is concentrated during the initial plastic phase shortly after hydration. This early rate of hydration generates considerable heat for a relatively short period of time. However, according to the teachings of the present invention, this high initial heat of hydration is dissipated well prior to final setting of the cement thereby reducing thermal cracking in the finished product.
It is a further additional object of the present invention to provide methods for producing rapid hardening high early strength portland-type cement compositions which achieve early high strength through the advantageous utilization of combined hydrated ettringite.
It is also an object of the present invention to provide methods for producing clinkered materials using processes that significantly reduce the environmental damage associated with cement manufacturing. These improved methods will result in a reduction in SO.sub.x on the order of 98%, a 35% reduction in NO.sub.x, and a 50% reduction in CO.sub.x as compared with conventional clinkered manufacturing methods. Furthermore, the previously unusable waste product, phosphogypsum, can be consumed by processes of the present invention, further reducing environmental impact.
It is yet another object of the present invention to provide early setting ultra-high early strength cement compositions at reduced costs and with greater manufacturing convenience.